Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace with two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products leave the pyrolysis furnace for further downstream processing, including quenching.
Quenching effluent from a heavy feed cracking furnace has been technically challenging. Most modern heavy feed furnaces employ a two-stage quench, the first stage being a high pressure 10340 to 13800 kPa (1500-2000 psia) steam generator and the second stage utilizing direct oil quench injection. See, e.g., U.S. Pat. No. 3,647,907 to Sato et al., incorporated herein by reference. In the 1960's high pressure steam generating cracked gas coolers deployed as transfer line exchangers were found to be especially useful in cracking liquid feeds. The high steam pressure (8100 to 12200 kPa (80 to 120 bar)) and high tube wall temperatures (300° C. to 350° C.) limited the condensation of heavy hydrocarbons and attendant coke formation on tube surfaces. Typically, boiler feed water preheating is effected within the convection section of the furnace.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstock such as crude oil and atmospheric resid, also known as atmospheric pipestill bottoms. Crude oil and atmospheric resid contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile, heavy ends of these feedstocks may lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatiles can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Additionally, some naphthas are contaminated with crude oil during transport. Conventional pyrolysis furnaces do not have the flexibility to process resids, crudes, or many resid or crude contaminated gas oils or naphthas that contain a large fraction of heavy non-volatile hydrocarbons.
The present inventor has recognized that in using a flash to separate heavy non-volatile hydrocarbons from the lighter volatile hydrocarbons which can be cracked in the pyrolysis furnace, it is important to maximize the non-volatile hydrocarbon removal efficiency. Otherwise, heavy, coke-forming, non-volatile hydrocarbons could be entrained in the vapor phase and carried overhead into the furnace creating coking problems in the convection section. It has also been recognized that the heated liquid bottoms produced from such flashing typically must be cooled, thereby providing an opportunity to enhance thermal efficiency of the steam cracking process.
U.S. Pat. No. 4,233,137, which is fully incorporated herein by reference, discloses a quench exchanger system which recovers heat from pyrolysis furnace cracked effluent in the form of high pressure steam by direct oil quench to 300° C.-400° C., followed by indirect heat exchange of the effluent/oil mixture in a shell-and-tube exchanger to transfer the heat into a high pressure water to obtain high pressure steam (40 to 100 kg/cm2).
U.S. Pat. No. 3,617,493, which is fully incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230° C. (450° F.) and 590° C. (1100° F.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
Co-pending U.S. Publication No. 2004/0004022 A1, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks, and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water. The bottoms from the flash can be cooled.
U.S. patent application Ser. No. 60/555,282, filed Mar. 22, 2004, which is incorporated herein by reference, teaches the use of steam generating quench exchangers with a furnace which includes a convection section vapor/liquid separator for removing non-volatiles from heavy feedstock. A steam superheating bank in the convection section can be located between a) the outlet for partially vaporized feed from the convection section before the vapor/liquid separator, and b) the inlet for reintroducing vapor to the convection section from the vapor/liquid separator.
It is known to produce high pressure steam from pyrolysis effluent using quench exchangers. U.S. Pat. No. 4,614,229, incorporated herein by reference, utilizes a primary non-liquid washed steam superheating transfer line exchanger and a secondary liquid washed transfer line exchanger steam generator to generate 10400 kPa (1500 psia) steam.
In using a flash to separate heavy liquid hydrocarbon fractions containing resid from the lighter fractions, which can be processed in the pyrolysis furnace, it is important to effect the separation so that most of the non-volatile components will be in the liquid phase. Otherwise, heavy, coke-forming, non-volatile components in the vapor are carried into the furnace causing coking problems.
During flashing to separate heavy liquid hydrocarbon fractions containing resid from the lighter fractions, which can be processed in the pyrolysis furnace, it would be desirable to cool the liquid bottoms fraction in such a way as to efficiently recover their heat. Accordingly, it would be desirable to provide a process for cooling liquid phase materials, e.g., bottoms taken from a flash drum used to separate heavy liquid hydrocarbon fractions containing resid from the lighter fractions, which can be processed in the pyrolysis furnace, while utilizing transferred heat to efficiently integrate the heat recovery in the overall furnace design.